In order to access certain types of hydrocarbons in the earth, it may be necessary or desirable to drill wells or boreholes in a certain spatial relationship with respect to one another. Specifically, it may be desirable to drill a borehole such that it has a specific location relative to a previously drilled borehole. For example, heavy oil may be too viscous in its natural state to be produced from a conventional well, and, thus, an arrangement of cooperative wells and well features may be utilized to produce such oil. Indeed, to produce heavy oil, a variety of techniques may be employed, including, for example, Steam Assisted Gravity Drainage (SAGD), Cross Well Steam Assisted Gravity Drainage (X-SAGD), or Toe to Heel Air Injection (THAI). While SAGD wells generally involve two parallel horizontal wells, X-SAGD and THAI wells generally involve two or more wells located perpendicular to one another.
X-SAGD and THAI techniques function by employing one or more wells for steam injection or air injection, respectively, known as “injector wells.” The injector wells pump steam or air into precise locations in a heavy oil formation to heat heavy oil. One or more lower horizontal wells, known as “producer wells,” collect the heated heavy oil. For an X-SAGD well pair including an injector well and a producer well, the injector well is a horizontal well located above and oriented perpendicular to the producer well. In contrast, for a THAI well pair including an injector well and a producer well, the injector well is a vertical well located near and oriented perpendicular to the producer well.
Steam or air from an injector well in an X-SAGD or THAI well pair should be injected at a precise point in the heavy oil formation to maximize recovery. Particularly, if steam is injected too near to a point of closest approach between the injector well and the producer well, steam may be shunted out of the formation and into the producer well. Using some conventional techniques, the point of closest approach between the two wells may be difficult to locate or the location of the point of closest approach may be imprecise.
Moreover, the relative distance between the injector and producer wells of an X-SAGD or THAI well pair may affect potential recovery. The wells should be located sufficiently near to one another such that heavy oil heated at the injector well may drain into the producer well. However, if the wells are located too near to one another, steam or air from the injector well may shunt into the producer well, and if the wells are located too far from one another, the heated heavy oil may not extend to the producer well.
Using many conventional techniques, it may be difficult to accurately drill one well in a specified relationship relative to another well. Indeed, standard measurement while drilling (MWD) direction and inclination measurements are usually too inaccurate to maintain proper spacing and relative positioning between two wells over their entire lengths. In part, this is because the location of each well becomes more uncertain as the length of the well increases. For example, the uncertainties may be represented as ellipses at different well lengths that represent the area in which the well may be located at a particular point. These ellipses increase in area with drilled depth. Thus, it is very difficult to accurately position wells relative to one another. Indeed, if the ellipses for a pair of wells overlap, there is potential for a collision between the wells. For these reasons, a standard practice is to use magnetic ranging to position one well with respect to another (e.g., a SAGD pair). However, magnetic ranging can be challenging for certain applications. For example, it may be difficult and/or expensive to use magnetic ranging when two wells are to be placed a relatively large distance from one another.